1. Field of the Invention
The present invention relates to a pn varactor that uses a pn junction as variable capacitance capacitor. More particularly, it relates to a pn varactor that has a small resistance and can cope with being built in an integrated circuit of a circuit that uses LC resonance.
2. Description of Related Art
As an example of a circuit employing a varactor, a voltage control oscillation circuit (hereinafter referred to as “VCO circuit”) can be mentioned. FIG. 10 shows a typical structure of the VCO circuit. This VCO circuit contains two varactors in an area surrounded by dotted line. Then, a control voltage VT is applied to their joint section. Thus, the capacity can be adjusted by adjusting the control voltage VT. For the reason, the VCO circuit is capable of controlling LC resonance frequency according to the control voltage VT. Generally, in such a VCO circuit, a ratio between carrier and noise (expressed in an absolute value, which is referred to as “C/N ratio” hereinafter) is preferred to be as large as possible.
However, the aforementioned conventional technology has following problems. That is, the C/N ratio of the VCO circuit cannot be increased so much. The reason is that a resistance of the circuit is large. Because the resistance is large, Q-value is small so that the C/N ratio is small. In order to improve the C/N ratio, it can be considered to reduce Kv (a ratio of a change amount of resonance frequency with respect to the change amount of the control voltage VT). If it is intended to achieve this with the characteristic of the varactor, it is recommendable to use a varactor having a mild change in capacity with respect to a change of the control voltage VT. However, generally, this kind of the varactor is not so an effective resolution because its own Q-value is so small. Further, a resistance of wiring section between the two varactors cannot be neglected. Thus, this is an obstacle to a trial for building VCO circuits in an integrated circuit.